1. Field of the Invention
The present invention relates to a method of deriving a standard 12-lead electrocardiogram for diagnosing heart disease including ischemic heart disease, acute myocardial infarction, and others by using a reduced number of electrodes attached to predetermined positions on the surface of a living body, and a electrocardiogram monitoring apparatus using the electrocardiogram derivation methods.
2. Related Art
To detect, measure, and record a conventional 12-lead electrocardiogram from a subject in a hospital or the like, a total of ten electrodes are attached to the surface of the subject""s body; six positions on the chest and four positions at the limb. Heart potentials sensed by those ten electrodes are input to an apparatus, namely electrocardiograph. The electrocardiograph produces twelve waveforms; six waveforms of limb leads, named as I, II, III, aVR, aVL, aVF, and six waveforms of chest leads named as V1, V2, V3, V4, V5, and V6.
The 12-lead electrocardiogram is a standard in diagnosing heart disease. It is especially important for diagnosing ischemic heart disease and acute myocardial infarction, where waveform changes such as the T wave and the ST segment changes in all leads have to be examined at the same time. Because ten electrodes are required to record 12-lead electrocardiogram, the recording and/or monitoring are usually performed in an environment where the subject is kept quiet. In many practical situations, however, mounting and maintaining such ten electrodes to record the 12-lead electrocardiogram are almost impractical. One example is ambulatory monitoring. It is also very difficult to do this for long-term bedside monitoring, because the number of electrodes and configuration severely restrict the mobility of the patient. Technically, most monitors use telemetry and at least eight channels of signal are necessary to be transmitted for the 12-lead electrocardiogram. This is not only expensive in cost, but also sometimes impossible because of the restriction of capacity in telecommunication. For example, most current telemonitors used in an ambulance usually have only one channel to transmit the electrocardiogram, which may be sufficient for diagnosing arrhythmias, but insufficient for diagnosing ischemic heart disease and acute myocardial infarction that are main causes of heart death. To put it into a nutshell, the problem is that there is a necessity to record and monitor the 12-lead electrocardiogram, but there is no means to mount and maintain such ten electrodes in many practical circumstances such as ambulatory monitoring, long-term bedside monitoring, homecare monitoring and so on.
To meet the need of monitoring 12-lead ECG in cases where mounting and maintaining ten electrodes are difficult, there is proposed a means called EASI lead system and EASI electrocardiogram (U.S. Pat. No. 4,850,370, and G. E. Dower, xe2x80x9cEASI-lead electrocardiography, Totemite Inc. Point Robeerts, Wash., 1996). The EASI lead system includes five electrodes: electrode E on the lower sternum, electrode A on the left axilla, electrode S on the upper sternum, electrode I on the right axilla, and an additional grounding electrode. The potential differences of A-I, E-S and A-S are recorded and the 12-lead electrocardiogram is calculated with coefficients developed by the inventor.
However, there are some difficulties for the electrocardiogram to be widely accepted by the clinical practice. An inherent limitation is due to the fact that all the 12-lead waveforms of the EASI electrocardiogram are calculated ones, not directly recorded ones. That means the EASI electrocardiogram only provides indirect, or secondary information. In clinical practice, most physicians generally do not trust secondary information for diagnosis. In addition, relevant laws in most counties prohibit using such estimated information for diagnostic behavior. Furthermore, most medical persons are not familiar with the EASI electrocardiogram. In any event, it is a fact that the EASI electrocardiogram is not easily accepted in clinical practice.
With ten electrodes placed in positions as shown in FIG. 1 and FIG. 2, the 12-lead electrocardiogram is defined as follows.
Accordingly, an objective of the present invention is to provide a method of deriving a standard 12-lead electrocardiogram and a 12-lead electrocardiogram monitoring apparatus in which the 12-lead electrocardiogram measurement uses a reduced number of electrodes for sensing the body surface potentials on positions of a subset of the standard 12-lead system, so that a portion of the resultant 12-lead waveforms are obtained directly from measured signals, as the direct or primary information for diagnosis, and others are derived from the measured signals, as the secondary and auxiliary information for improving the accuracy of diagnosis.
According to an aspect of the present invention, there is provided a method of deriving a standard 12-lead electrocardiogram comprising the steps of: attaching a plurality of electrodes on positions of a subset of the standard 12-lead system used in routing rest 12-lead electrocardiogram testing, or a subset of another standard lead system, called M-L lead system, used in routing exercise 12-lead electrocardiogram testing; sensing and measuring the body surface potentials from the plurality of electrodes; calculating waveforms of unmeasured leads of the standard 12-lead lead systems; and constructing the 12-lead electrocardiogram in which a portion of waveforms are obtained from directly measured signals and the other portion of waveforms are derived by the calculation.
In the method of deriving a standard 12-lead electrocardiogram, an array, called heart vector, is obtained, as an intermediate quantity, based on measured signals of leads and the values of lead vectors of the measured leads, which are available according to publications of Frank (E. Frank, xe2x80x9cThe image surface of a homogeneous torso,xe2x80x9d Am Heart J, 47: 757-768, 1954). The heart vector is calculated for each time instant of sampling. The waveforms of unmeasured leads in the standard 12-lead system are derived using the heart vector together with values of lead vectors of the unmeasured leads, which are also available according to publications of Frank as mentioned.
In the method of deriving a standard 12-lead electrocardiogram, the said plurality of electrodes can include RA, LA, RL, LL, V1, and V6 of the standard 12-lead system (FIG. 1) used in said routing rest 12-lead electrocardiogram testing, or of said M-L lead system (FIG. 2) used in routing exercise testing; the said plurality of electrodes can also include RA, LA, RL, LL, V1, and V5 of the standard lead system used in said routing rest 12-lead electrocardiogram testing, or of said M-L lead system used in routing exercise testing;
In the method of deriving a standard 12-lead electrocardiogram, the waveforms of limb leads and measured chest leads are obtained according to Table 1. The remaining unmeasured chest leads are derived based on said method.
According to another aspect of the invention, there is provided an electrocardiogram monitoring apparatus using the method of deriving a standard 12-lead electrocardiogram of the present invention, comprising; a plurality of electrodes attached to the surface of a living body, for sensing and measuring electrocardiographic potentials on positions of a subset of the standard 12-lead system; a potential detector for detecting the potentials from said electrodes; an operational amplifier for amplifying the potential signals and obtaining waveforms of leads from said potentials detector; a 12-lead electrocardiogram operation unit for synthesizing and calculating waveforms of undetected leads of the standard 12-lead system using data of waveforms of said obtained leads from the amplifier; and a 12-lead electrocardiogram display unit for displaying 12-lead electrocardiogram waveforms received from said 12-lead electrocardiogram operation unit.